Firefox ignores mime header for ogg

I have a web application that builds the audio element in the browser for files on a server. That is working fine for mp3, and it is working in Chrome also for ogg. In Firefox (35.0) however Firefox seems to think the served ogg is a mpeg and thus does not play it:

Notice how the "type" in the network inspector says mpeg, the content type however correctly says audio/ogg. Did someone stumble upon this problem before and knows how to let Firefox correctly identify the .ogg?

On other pages this FF has no problem playing ogg, it has to be something very specific.